The art of controlling the magnitude and the directionality of the physical and mechanical properties of polymers has been a subject of considerable interest to polymer engineers in recent years. Whereas the magnitude has been the basic objective of numerous molecular deformation processes by which the tensile properties can be enhanced significantly, as a result of the chain orientation and extension in some particular direction, usually the machine direction, the directionality has been traditionally addressed by the construction of fiber-reinforced composite structures. In conventional meltprocessing, where the objective is the conversion of a polymer raw material into a solid product of some specific shape, the traditional approaches of controlling the thermomechanical history of the polymer melt have not been efficient enough for controlling the properties of the final product. The reason is that some of the chain orientation and extension obtained (produced in the feeding and/or processing zones) has tended to relax through an uncontrolled molecular relaxation process before the polymer has been completely solidified in the forming and/or shaping stage. The basic objective of obtaining a product of some specified shape has been met, but such a product has had non-isotropic properties.
Homogeneous polymer structures (i.e., structures without fillers or fiber reinforcement) with enhanced mechanical properties in planar directions have been produced recently This has been done by induced crystallization under curvilinear flow conditions and by solid-state-forming processes. This has resulted in products with multiaxial chain orientation and extension and products with an overall biaxial (but unbalanced) chain orientation and extension. The first approach is more suitable for processing readily melt-processable polymers by rapid output processes, whereas the latter approach can be used also for processing more intractable polymers, such as ultra-high-molecular-weight polyethylene.
For example, a solid-state-forming process of Dow Chemical (known as the SPF Process), is a process for the development of containers with biaxial orientation; the SPF Process comprises the forging of a lubricated polymer preform or a briquette, at a temperature between the softening point and the melt point of the polymer, into a sheet preform, which is automatically clamped and cooled at its periphery and then plug-assist and pressure-formed into a cooled mold to the shape of the finished article (the container). Although the overall orientation of the product is biaxial and results in the substantial enhancement of its mechanical properties, the way it is generated is unbalanced and cannot be controlled to vary topologically in a non-symmetrical fashion (i.e., unlike the deep-draw process).
Similarly, the solid-state-forming process disclosed in U.S. Pat. No. 4,747,990 involves the shaping of a polyethylene resin in a closed-mold configuration; the deformation of the polymer (and hence the degree of chain orientation) is non-uniform in different parts of the product, e.g., of the material close to and against the plunger versus the material against the lower part of the mold cavity.
Both the SPF Process and the process disclosed in U.S. Pat. No. 4,747,990, as well as other solid-state-forming processes, e.g., the matched mold forging process, operate under "closed mold" conditions. In the SPF Process the outer clamping ring on the periphery of the mold provides a physical constraint to the maximum deformation that one can deform the polymer under compression during the forging step for the fabrication of the sheet preform. Similarly, in the other processes, e.g., the process of U.S. Pat. No. 4,747,990, the maximum deformation is controlled by the ratio of the polymer preform dimensions to the mold dimensions.
In addition, many applications may benefit from the development of products with enhanced planar mechanical properties and these require good dimensional stability.
In the case of ultra-high-molecular-weight polyethylene it is very difficult to control this important parameter: first, because this polymer is dimensionally unstable, even during a simple machining operation; second, because when this polymer is oriented, it exhibits a remarkable springback, which also affects the magnitude of the properties of the final product. (Springback refers to the tendency of a formed product to revert partially to its original configuration after removing a compression or tension load, and it is a major concern in solid-state-forming operations.)
Thus, the fabrication of ultra-high-molecular-weight polyethylene products to achieve enhanced mechanical properties and good dimensional stability is a challenging task. Also, it is important to realize that polymer structures with enhanced planar mechanical properties can be used at substantially reduced thicknesses, thus reducing the unnecessary bulk of material that is currently used in many applications for increasing their load-bearing performance. For example, in the case of orthopaedic prosthetic products, e.g., acetabular liner and tibial plates, their thickness can be reduced without sacrificing their mechanical performance.
An object of this invention is the fabrication of a molded ultra-high-molecular-weight polyethylene product with enhanced and balanced planar mechanical properties and controlled dimensional stability.
Another object of this invention is the fabrication of such an ultra-high-molecular-weight polyethylene product by using an "open mold" configuration.
Another object of this invention is the compression molding of an ultra-high-molecular-weight polyethylene to a deformation ratio which is unrelated to the ratio of the physical dimensions of the polymer preform and the mold cavity and, in contrast, depends only on the material properties of the polymer under the employed processing conditions.
A further object of this invention is the fabrication of wear-bearing ultra-high-molecular-weight polyethylene products of thinner load, having enhanced mechanical properties.